HP 8116A 001 no waveforms below 10Hz

[Mick B]

Hello , I'm looking for a little help I just got an HP8116A and I have myself thoroughly lost in the manual. The problem Is no wavesforms below 10Hz I have the service manual and tried to trace this out, But not much diagnostic help for problems <10Hz ( That I have found) but plenty if the issue is above >10Hz to 50Mz.
 I have test equipment Hantek scope, HP counter, HP spectrum analyzer, Fluke 87. If some one could point me in the right direction. Thanks Mick A BIG TIME NUBE!

[MarkL]

According to the service manual (on page 10.3-4 in my version):

  Range Switches
 
  The ramp timing capacitance is selected from range latch U210.
  Frequencies from 1 mHz to 10 MHz are possible with the external
  capacitors, frequencies > 10 MHz use the internal capacitor in
  U201. Note that a capacitance amplifier, located on control
  board A2, is used to provide the high capacitances required at
  frequencies < 10 Hz.

If everything else is working fine (including verifying the power supply voltages), I would start looking for proper operation of this capacitance amplifier.

There is a theory of operation for it in section 10.5, and the circuit itself is on schematic sheet 2A.

[Smith]

Does it have the option 001, external input? I've got one that did. It would not produce low frequency outputs on a regular basis. It would randomly start, and randomly cut out. It actually was the connector for the external input. Normally when the connector (think it was a 3.5mm jack plug style)  is pulled out the connector would pass trough the enable, thus always enabling the output when the connector was not used. Mine was a little corroded, so the enable signal would stop. I replaced it and it has been working fine since. Took me quite some time as this particular option was not shown in the main schematics in the service manual I used, it was shown in some addendum I read later.

[Mick B]

Thank you for your reply. I drew the same conclusions you did. Luckily Smith said to look at the 3.5mm jack. and that was the problem! Now I want to investigate why the (Option 001) Hold Input hold only affected <10hz. I'm just glad it wasn't the capacitance amp not much information on that except the location It almost seamed like checking ALL the rest of the functions had to work to determine the CAP AMP was having a problem. Like i said I'm a nube and trying to understand ELC, latches, Dac's, address decoders etc. AND not finding the caps for the AMP, and flipping through pages in the service manual on 3- 27" was still wearing me out I don't see how someone could do this on 1 monitor. Thanks again for your input.       

[Mick B]

Smith. THANK YOU, THANK YOU!!! That was the problem, saved me so much grief. Do you remember what led you to look there? I think It would take a long time to find some correlation between the problem. and the fix. Thank you once again. Mick

[MarkL]

The "Hold Input" jack is connected directly to the capacitance amplifier circuit, so in examining that circuit for correct operation you would have found it.  But that was a good experienced insight from Smith; glad you got it working!

[Smith]

I'm glad you got it working!

The unit I got (I actually got another later) was deemed BER, so I could have it. It took me quite some time, but I remember there was some starting of enable condition for low frequencies. The input was shown, but not explained anywhere. Later on I found the input shown in a schematic of the option, that was where everything fell together. It was one of my first "more difficult" issues way way back. It wasn't helping these units sold on the internet for over 2K dollar at the time, so there weren't a lot of home users being able to help.

The second one I got years later had the trigger output blown. The transistor was a HP branded one that had become unobtainium. There was no equivalent to be found on the internet. I tested and parametrized a working one from another stage, and parameters where equivalent of an 2N3904 if I remember correctly (was 5-10 years ago). I placed it, and it worked fine again.

PS If your buttons are sticky (feeling tacky when pressed) you can take off the front and clean them by rinsing them with IPA while switching them a few times. This took care of both of my units. Calibration of the thing literally drove me mad, until I replaced all the internal trimmer pots of both with multi turn ones. Cost me a pretty penny, but it was worth it.

[Mick B]

MarkL You are correct! I missed this on chip U3 (74LS173N) When either M or N (or both) is (are) high, the output is
disabled to the high-impedance state; however, sequential
operation of the flip-flops is not affected. When that jack is open it goes high. Thanks again. Mick

[Mick B]

Smith, Thanks again. Yep when I got this all the button's were sticky and and I did IPA thing freed them right up. I had this one for a couple of months before I noticed the <10Hz issue. Seeing how these units are not precision interments and the output are still within specs. abet some on the edge. I'm going to forgo the CAL until it's absolutely necessary. Perhaps I will seek your advice again in the future. take care. Mick     

[MarkL]

...
I'm going to forgo the CAL until it's absolutely necessary.
...

I would agree with that.  Leave it alone if you're happy with it.

The calibration procedure on these units is rather laborious.  It took me the better part of a day to complete it on one unit, and at the end the procedure in the manual is actually incomplete, although you can infer what was meant.  (Maybe it was my copy of the manual, but it's an original paper copy and all the PDFs on the net end the same way.)  Also, many of the adjustments interact with each other, and some are extremely sensitive to even a squeak of a turn.  It will drive you crazy.